Conventionally, there is a method of measuring adhesion of adhesive sheets, such as sticker labels, adhesive tapes, adhesive papers, or the like, which is constituted by adhering a sampled adhesive sheet to a stable surface of the substrate by securing a free end of the sampled sheet to anchor means of a tension tester, and then allowing the sampled sheet to be peeled off from the substrate so that the tester can measure the strength needed for peeling off the sampled sheet from the substrate.
Thus, a tension meter is conventionally used for measuring adhesion, but the tension meter is not inherently suited for measuring adhesion, because it is rather a stationary apparatus for rating the tensile strength of material, so that the tension matter cannot portably be transferred to the desired location. Consequently, the operator incurs much inconvenience in execution of adhesion tests in such a desired location as an adhesive-sheet manufacturing facility or in any work site where adhesive sheets are used.
Actually, there are two ways of measuring adhesive strength of adhesive sheets. The one is the 90.degree.-peeling method which provides measurement of the strength needed for peeling off the adhesive sheet by pulling its free portion in the direction perpendicular to the adhered surface or the substrate surface. The other is the 180.degree.-peeling method which provides measurement of the strength needed for peeling off the adhesive sheet by pulling the free portion in the direction in parallel with the adhered surface upon turning up the free portion of the adhesive sheet from the adered surface.
Generally, the tensile strength needed for peeling off the adhesive sheet from the adhered surface contains a variety of factors including the following: (1) the angle between the direction of pulling the free portion of the adhesive sheet and the sheet-adhered surface of substrate, i.e., the peeling angle itself, (2) the rate of elasticity of adhesive agent, which is variable by the peeling angle, and (3) rigidity of adhesive sheet, which is also variable by the peeling angle, i.e., elasticity and yielding stress against bending. Consequently, in order to strictly determine the peeling strength of an adhesive sheet, in principle, the tester should fully determine the peeling strength for all angles when executing peeling operations, and yet the established angle should be held constant throughout the operation for measuring angle of peeling the adhered specimen.